In order to reduce the weight of the body of an automobile, use of aluminum-alloy materials also for automotive heat exchange parts in place of conventionally used copper-alloy materials has been increasing. Then, for the aluminum-alloy materials for heat exchange parts, there have been used corrosion-resistant aluminum-alloy materials including a multi-layered sheet (which may also be referred to as a clad sheet or a clad material).
When such a multi-layered sheet is brazed to be assembled into a heat exchanger, the multi-layered sheet is formed into a brazing sheet including a core layer made of aluminum alloy on one side of which a sacrificial layer (sheet) is cladded, and on the other side of which a braze clad layer is cladded.
FIG. 4 shows an example of a heat exchanger (radiator) for an automobile made of aluminum alloy. As shown in FIG. 4, in a radiator 100, generally, between a number of radiator tubes 111 made of aluminum alloy and in the form of flat tubes provided therein, a heat radiation fin 112 made of aluminum alloy and processed in corrugated form is formed integrally. Opposite ends of the tubes 111 are configured to respectively open into the spaces formed by headers 113 and tanks (not shown). With the radiator 100 in such a configuration, a heated refrigerant is fed from the space of one tank through the inside of each tube 111 into the space on the other tank side. Thus, in the portions of the tubes 111 and the heat radiation fin 112, heat exchange is performed, so that the cooled refrigerant is circulated again.
The tube 111 made of an aluminum-alloy material includes a brazing sheet 101 made of aluminum alloy. FIG. 5 shows a cross section of the brazing sheet 101 made of aluminum alloy. In FIG. 5, the brazing sheet 101 is configured such that, on one side surface of a core layer 102 made of aluminum alloy, a sacrificial layer made of aluminum alloy (which is also referred to as a coating material) 103 is stacked (cladded), and on the other side surface of the core layer 102, a braze clad layer 104 made of aluminum alloy is stacked (cladded). Incidentally, in the case of the clad sheet made of aluminum alloy, the sheet is formed as a multi-layered sheet including only the sacrificial layer 103 cladded on one surface thereof.
Such a brazing sheet 101 made of aluminum alloy is formed into a flat tube by a forming roll or the like, and undergoes electro-resistance welding or brazing heating. As a result, the brazing sheet 101 itself is brazed, resulting in formation of the fluid passage as with the tube 111 of FIG. 4.
The main component of the refrigerant (coolant) of the radiator is a water-soluble medium. A refrigerant containing this and appropriately a commercially available corrosion inhibitor and the like is used. However, when such a refrigerant is used, by an acid formed upon deterioration with time of the corrosion inhibitor and the like, the aluminum-alloy materials such as the sacrificial layer and the core layer unfavorably become more susceptible to corrosion. For this reason, use of an aluminum-alloy material having a high corrosion resistance to the water-soluble medium becomes essential.
Therefore, for the core layer 102 made of aluminum alloy for use in a multi-layered sheet of a brazing sheet or a clad sheet, from the viewpoints of corrosion resistance and strength, there is used an Al—Mn series (3000-series) alloy, such as 3003 which includes a composition such as Al-0.15 mass % Cu-1.1 mass % Mn specified in JIS H4000. Whereas, for the sacrificial layer 103 normally in contact with the refrigerant, alloys of Al—Zn series such as 7072 including a composition of Al-1 mass % Zn, or Al—Zn—Mg series (7000-series) are used aiming at preventing corrosion and increasing the strength by Mg diffusion into the core layer 102. Further, for the braze clad layer 104, there is used an Al—Si series (4000-series) alloy such as 4045 including a composition such as Al-10 mass % Si which is low in melting point.
The radiator 100 is assembled integrally by brazing using the tubes 111 formed using such a brazing sheet 101, the heat radiation fin 112 subjected to corrugate processing, and other parts. The brazing methods include a flux-brazing method, a Nocolok brazing method using a non-corrosive flux, and the like. Thus, brazing is performed by heating to a temperature as high as around 600° C.
In the radiator 100 thus assembled, particularly, in the tubes 111, the liquid refrigerant which is at from high temperatures to low temperatures, and high pressures to normal pressures always flows/circulates. Namely, the tubes 111 are repeatedly applied with stresses such as fluctuations in internal pressure thereof and vibration of the automobile itself over a long time. Accordingly, the tubes 111 are required to have fatigue properties withstanding them. If the fatigue properties are low, and fatigue fracture occurs, the fatigue fracture occurs as a crack of the tube 111, and develops to penetrate through the tube 111. This causes leakage of liquid from the radiator. For this reason, the improvement of the fatigue properties of the radiator tube is an important problem.
Conventionally, various improvements of the fatigue properties of the radiator tube have been proposed. For example, Patent Document 1 is intended to attain the following. The core layer in the brazing sheet made of aluminum alloy is an aluminum alloy including Cu, Ti, and Mn, and regulated on Si, Fe, and Mg. The mean grain size L in the rolling direction in the longitudinal section of the core layer is set at 150 to 200 μm, thereby to improve the corrosion resistance of the weld part of the tube. As a result, the fatigue fracture property due to repeated bending of the tube, i.e., the vibration fatigue resistance under automotive vibration is improved. Patent Document 2 is intended to attain the following. The mean grain size in the direction of thickness on the sacrificial layer side is set at less than the thickness of the sacrificial layer. This improves the corrosion resistance of the sacrificial layer. As a result, the fatigue fracture property due to the repeated bending of the tube, and repeated internal pressure load, i.e., the fatigue properties are improved.
Further, it is generally known that the fatigue properties are related to the static tensile strength. Also for the heat exchanger, for example, as in Patent Document 3, there is proposed a material including Cu added therein in order to improve the tensile strength of the material. Then, Patent Document 4 is intended to improve the vibration fatigue resistance by the improvement in microstructure. Namely, in Patent Document 4, in a heat exchanger using an aluminum-alloy brazing sheet of a three-layer construction in which a Cu-containing aluminum-alloy core layer, an aluminum-alloy braze clad layer, and a Zn- and Mg-containing aluminum-alloy sacrificial layer are cladded, the following is proposed. In the heat exchanger, in the core layer side interface part in the vicinity of the interface between the core layer and the sacrificial layer of the brazing sheet after brazing, specific Al—Cu—Mg—Zn series precipitates are distributed. This is intended to attain the following: the strength of the core layer side interface part is enhanced by age-hardening due to Al—Cu—Mg—Zn series precipitates; this improves the fatigue fracture property due to a repeated internal pressure load, i.e., the fatigue properties.
Further, in Patent Document 5, a brazing sheet made of aluminum alloy includes a core layer of Al—Mn series alloy, a sacrificial layer such as Al—Zn series alloy cladded on one side surface of the core layer, and a braze clad layer of Al—Si series alloy cladded on the other side surface of the core layer. The texture of the brazing sheet is defined with the X-ray diffraction intensity ratio. In Patent Document 5, plastic deformation in a direction parallel with the rolling direction of the brazing sheet tends to uniformly occur. As a result, even when a tensile or compressive cyclic stress is applied in the rolling direction of the brazing sheet, deformation ceases to locally concentrate. This delays the development of cracks in the thickness direction, which can improve the fatigue life of the brazing sheet including the fatigue in the plastic region.
Other than these, in order to improve the corrosion resistance of not a brazing sheet but a heat radiation fin including the same 3000-series aluminum alloy, it is proposed that the shapes and the number densities of constituent particles and intermetallic compounds in the microstructure are defined (see, e.g., Patent Documents 6, 7, and 8). Corrosion of such a heat radiation fin leads to disappearance of the fin itself, and hence the corrosion resistance is important. For this reason, definitions of the shapes and number densities of constituent particles and intermetallic compounds in the microstructure described in Patent Documents 6, 7, and 8 are also linked to the technical problem characteristic of the heat radiation fin of the corrosion resistance improvement.    [Patent Document 1] JP-A-2003-82427    [Patent Document 2] JP-A-11-100628    [Patent Document 3] JP-A-10-53827    [Patent Document 4] JP-A-9-95749    [Patent Document 5] JP-A-2006-291311    [Patent Document 6] JP-A-9-78168    [Patent Document 7] JP-A-2000-119783    [Patent Document 8] JP-A-2005-139505